"By the time Auckland gets rail to the airport… [Bill English] be 86, I'll be 66 and Todd Barclay will be into his teens," the Labour deputy quipped, drawing howls of laughter from the House.

Mr Barclay, MP for Clutha-Southland, might look young - but he's actually a wizened 26 years of age.

Rising above the din was Ms Bennett.

"That was hilarious!" the Upper Harbour MP bellowed.

But she wasn't being sarcastic, saying Ms Ardern's wisecrack took her by surprise.

"She doesn't usually tell jokes," Ms Bennett told The AM Show on Friday.

"It is funny that she told one, and then she kind of sits down and [doesn't] laugh. I'm one of those people who laughs before I tell a joke, which is really annoying, and she's one of those people that deadpans. She did it brilliantly."

But Ms Ardern said she couldn't take all the credit.

"It was a collaborative joke. It was a joke by committee, I'm gonna be honest."

And don't expect it to happen again anytime soon.

"I sat down and then stressed whether or not I had offended Todd."

The Government last month confirmed plans to build a rail link to Auckland International Airport - but not in the next 30 years - by which time Mr Barclay will still be a decade away from being eligible for the pension.